The present invention relates to gas generating systems for vehicle airbags and, more particularly, to a gas generating system incorporating a mechanism for maintaining system combustion pressure at a value within a predetermined range.
Many solid propellants have an optimum pressure range for combustion. It can be difficult to maintain the system internal pressure within the optimum pressure range during the majority of the combustion reaction. For example, in degressive-burning propellants, the surface area of a propellant grain decreases as the propellant burns. As the grain surface area decreases, the gas generation rate correspondingly decreases. For a gas generating system in which the total aperture area for gas exit flow from the system housing is constant, a decrease in the gas generation rate will result in a decrease in housing internal pressure, thereby correspondingly reducing propellant combustion pressure. Eventually, a point is reached where the combustion pressure falls below the desired pressure range for optimum combustion.
In addition, low-pressure combustion of the propellant outside of the optimum pressure range may increase the generation of undesirable effluents. Furthermore, operating outside of the optimum combustion range may adversely affect the combustion, thereby abbreviating or shortening the burn of the propellant, or inhibiting sustained combustion of the propellant.
Thus, it is desirable to maintain the system internal pressure within the optimum range for combustion of the propellant for as much of the combustion reaction as possible. The present invention is directed to reducing the total gas exit orifice area available for gas exit flow as the gas generation rate decreases, thereby maintaining optimum combustion pressure and reducing the generation of undesirable effluents.